Many houses, stores, and factories across the U.S. now have arrays of photovoltaic solar cells mounted on their roofs. These days, that’s hardly news.

But Ford’s announcement that it will install half a megawatt of power-generating solar panels at its Wayne, Michigan, assembly plant has a new angle: The plant will also store some of that solar energy in lithium-ion batteries, letting Ford draw on it even after dark.

Pairing lithium-ion storage batteries–which use large arrays of the same lithium-ion cells that power future plug-in hybrid and electric cars–is seen by utilities as a way to give solar energy more flexibility, making it useful beyond sunny daytime hours.

100 homes or 38 Tesla packs

It’s not a huge system: 2 megawatt-hours of storage could power 100 average homes in the state for a year. It’s roughly equivalent to 38 of the 53-kilowatt-hour lithium-ion battery packs in the 2010 Tesla Roadster electric sports car.

The energy generation and storage equipment is a joint project among Ford, local utility Detroit Edison, and energy-storage firm Xtreme Power.

The 500-kilowatt photovoltaic array and the 2-megawatt-hour storage system together will cost $5.8 million; funding comes from the utility, the Michigan Public Service Commission, and Ford. The company says it will save roughly $160,000 a year in energy costs once a smaller, second system is installed to power plant lighting systems.

The renewable energy provided by the solar system will feed into the power used to produce the new 2012 Ford Focus compact sedan and hatchback. The low-volume Ford Focus Electric model, which runs solely on battery power, will also be built at the Wayne plant.

Production of the Focus and Focus Electric starts next year, with a next-generation hybrid (likely the first Ford hybrid to use a lithium-ion battery pack) and a plug-in hybrid to follow in 2012.

“Game changer”

Pairing lithium-ion storage batteries–which may use large arrays of the same basic lithium-ion cells that power future plug-in hybrid and electric cars–is seen by utilities as a way to make solar energy far more flexible, making it useful beyond sunny daytime hours.

Ed Kjaer, electric transport director at Southern California Edison, has spoken often about the “game changer” that energy storage represents for utilities.

Aside from dams–essentially huge energy storage devices–all of the power that utilities provide must be consumed within a fraction of a second, because they have no way to store it. Cost-effective lithium battery packs could change that, leading to what Kjaer calls “Electric Utilities 2.0.”

Storing off-peak power

The Wayne plant’s energy storage system also allows Ford to draw power from the electric grid during off-peak night hours, when demand falls and prices are lowest. It can then be fed back into the plant during the day, reducing peak loads for Detroit Edison.

This isn’t the first Ford plant to use renewable power; its Dagenham Diesel Centre in the U.K. generates all its electric power from onsite wind turbines, and it has installed one of Europe’s largest grid-connected photovoltaic arrays at its Bridgend Engine Plant in Wales.

Meanwhile, drivers of the 2012 Ford Focus Electric may take pleasure in knowing that some small portion of the electricity used to build it came from the sun, rather than from petroleum-based sources.

Written by John Voelcker, this post originally appeared on GreenCarReports, one of VentureBeat’s editorial partners.